1. Field of Invention
The present invention relates to a method of fabricating a bonding pad for a semiconductor. More particularly, the present invention relates to a method for improving bonding pad surface quality.
2. Description of Related Art
In the manufacturing process of the bonding pad for an integrated circuit, the passivation layer over the wafer is patterned and then the bonding pad is exposed by forming an opening within the passivation layer. Thereafter, an alloy thermal process is performed to enhance the adhesion between the aluminum copper alloy on the surface of the bonding pad and the metal underlying thereof. Since the thermal expansion coefficient of the aluminum copper alloy is large, hillocks are created on the aluminum copper alloy by thermal stress in the metallic material due to the thermal alloying process. After the etching process for forming the opening in the passivation layer, a cleaning step performed in acid tank is required in order to remove residue on the wafer. However, the aluminum copper alloy is easily corroded by the acid solution to cause damage thereon. A polyimide is then coated over the wafer. When photolithography for the polyimide is carried out, the grain boundary of the aluminum copper alloy is attacked by the developer, leading to cracks. In addition, the aluminum copper alloy is subject to Al.sub.2 Cu phase formation, which easily reacts with the aluminum in the aluminum copper alloy to cause a galvanic reaction and accordingly, Al.sup.3+ is generated and leads to erosion.
Therefore, the appearance of the aluminum copper alloy on the surface of the bonding pad is seriously degraded when the alloying, cleaning and development processes are complete. However, the bonding pad is the only structure exposed by the passivation layer, and thus the appearance of the bonding pad is the only quality which the clients is capable of checking. As a result, the clients will reject the product due to the bad appearance of the bonding pad, even though no problems manifest during the wafer acceptance test (WAT). As a result, the producer who makes the bonding pad has to discard the wafers and loses substantial amounts of money.